1. Field of the Invention.
The present invention relates to the field of modulator demodulator sets and other data devices for digital communication over conventional telephone lines.
2. Prior Art.
Various types of modems, that is modulator/demodulator sets for digital communication over conventional telephone lines are well-known in the prior art. Such devices generally fall into one of two categories, specifically acoustic coupled devices and direct connect devices. Acoustic coupled devices may be characterized as having a speaker and microphone for disposition adjacent the microphone and speaker, respectively, on a conventional telephone handset, so that the digital signals being transmitted and received to and from the telephone system are coupled as audible signals. This of course works reasonably well, as a conventional telephone lines have a relatively narrow band pass which is not suitable for transmission of base band signals anyway. Acoustic coupling has the specific advantage of not requiring a direct line connection to the telephone line so as to be highly versatile, provided the coupler itself is suitable for mating with the desired range of telephone handsets. It does have the disadvantages, however, of sometimes being larger than a direct connect modem because of the requirement of the couplers and coupler spacing, of perhaps being more expensive for the same reason, and of suffering somewhat in signal-to-noise ratio because of the conversion of electronic signals to acoustic signals and back again, particularly in light of the poor frequency response of speakers and microphones in conventional handsets.
In recent years modular telephone connectors for connecting telephone lines to receivers and for connecting handsets to receivers have been mandated. These modular connectors allow the direct connection of a modem to a telephone line with the attendant reduction in cost and size of direct connect modems and improved performance over acoustic coupled modems. At the present time, a substantial percentage of telephones use the modular connectors, which percentage is steadily increasing. Accordingly, the ratio of direct connect modems to acoustic modems being sold is undoubtedly also steadily increasing because of the advantages of direct connects. However, there are many instances where direct connect modems cannot be used, such as in phone equipment already installed before the advent of the modular connection, multiline telephones, phones in phone booths, motels, etc. Thus, there will be instances encountered by traveling persons and others using portable terminals and other digital equipment where the direct connect modem commonly used therewith cannot be used with available phones. Heretofore there has been no known adapter or converter such as the present invention for providing an acoustic interface between a direct connect modem and a telephone to allow the use of a direct connect modem system on a phone system to which a direct connection cannot be made, whether that direct connect modem be a stand-alone device or integrated into some larger piece of digital equipment.